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how fast does a 747 have to go to stay up in the air

20K views 54 replies 13 participants last post by  RWSmith 
#1 ·
Just wondered how fast does a 747 or a plane of a similar size have to travel to stay up in the air , in other words whats the slowest it can go and still stay up ??
 
#5 ·
The take-off or "rotate" speed for airplanes differ depending on size, type, etc...

On average lets say most of your small single engine airplanes rotate at about 55 knots which equals about 60-65 mph. They then speed up during climb to about 80-90 knots (about 90-110mph). Cruise speed is anywhere from 100-140 knots (115-150 mph).

Now obviously as the airplane gets bigger it tends to get faster.

Your jet aircraft usually stay around the same speeds though. Rotate speed is about 130-150 knots (150-170 mph). They then cruise anywhere from 250-400 knots (290-460 mph). Such a big possibility for the speeds because it all depends on the airplane and the altitude.
 
#8 ·
A 747 will "take off" at around 180mph, this will depend on conditions such as weight, air temperature etc.

As altitude increases, density decreases so lift reduces; so

a 747 will stay in the air at 50ft at 180mph give or take, but as altitude increases the speed for lift to overcome gravity will increase.

For more information on this google Density Altitude.
 
#19 ·
Operating Performance:

Mmo 0.85 Mach Max. Operating Speed
Mne 0.92 Mach Never Exceed Speed
Vno knots Max. Operating Speed IAS
Vne365 knotsNever Exceed Speed IAS
Vat150 - 156 knotsLanding @ Runway Threshold Speed @ MLW full flap/Gear down
Take-off speed@ MTOW with Flap 3 is 155 - 160 kts, otherwise 155kts @ flap 2
ILS & Approach speed 180 - 150 kts @ Flap2 (Kyb 2)

* DO NOT Exceed 250kts @ or Below 10,000ft Altitude.*

Take-off speed@ MTOW:

V1 145-148kts @ flap2/3 (V1 is the go or no-go decision speed)
VR 155-160kts (VR is the rotation speed ie lift-off speed)
V2 180kts (V2 is the safe climb-out speed)
ILS & Approach speed @ MLW:

185 kts Flap2 (Kyb 2) to full flap/gear down @ Vat 150 - 155 kts
Vat = 1.30 Vso (Vso is stall speed @ full flap/gear down)
Cruise Speeds:

Cruise Speed 490 KTAS
Cruise Altitude 28,000 - 35,000ft depending on weight.
Typical Cruise Speed is 0.80 - 0.85 Mach @ FL280 - FL350
Long Range Cruise speed is 496 KTAS @ FL350 (35,000ft)
Max. cruising speed is 507 KTAS
 
#20 ·
A quick lesson

Don't confuse ground speed with air speed. Take-Off air speed for a 747 will be approximately 150 MPH but depending on air speed/pay load it maybe higher or lower.
Aircraft always try to take off with a head wind to utilise the air speed to considerably reduce ground speed to gain altitude. Take-Off with a tail wind will require a higher ground speed in order to generate enough lift to gain altitude.
Also don't let the term lift confuse you either as the aircraft isn't lifted it is pushed up by the higher density air under the wings created by the faster airflow over the top of the wings. On take off the pilot will use the flap to create even more lift by increasing the top skin area of the wings. On landing the pilot will use the flap on the trailing edge to slow the aircraft down in conjunction with the spoilers. He will also deploy the slats along the leading edge. The spoilers increase drag to slow the aircraft down whilst the slats and flap increase the lift efficiency of the wing so that it can fly at a lower air speed and remain airbourne. However, with increased drag the engines need to work harder, hence you will often hear the pitch of the engines change as the pilot slightly increases thrust to remain airbourne.
In flight the wings need to be as efficient as possible so the slats/flaps/spoilers are fully wound in to increase fuel efficiency.
Lesson over!!!!:smthumbup
 
#23 ·
On landing the pilot will use the flap on the trailing edge to slow the aircraft down
Not if they want to fly the aircraft ever again they won't!

You do not use flaps to reduce airspeed, you use flaps to change the lift / drag coefficient of the wing to produce greater lift for the same airspeed.

You must never use the flaps to slow down, they are jsut not designed for the load.
 
#24 ·
I've just phoned a mate of mine who is a First Officer for Virgin Atlantic, but his phone is off at the moment, (he's probably lying on some beach in St. Lucia with six girls from the cabin crew :smthumbup) Hopefully he will give us his opinion, if he doesn't know I suggest you all book BA next time you travel to the States or you could be swimming at some point during the journey.
 
#25 ·
flaps are used to lower the speed of the a/c and give the same amount of lift as without at higher speed. Its lift Vs speed. A 747 will use air brakes on the trailing edge of the wing usually on the upper surface to distort the airflow - this will slow it down these will be deployed once on the runway. But as Ben said, you don't use them to physically slow the a/c down, there's no need and it doesn't work that way.
 
#28 ·
So from your PM:-

RINGREY said:
Just worked in the industy for the best part of 15yrs on the most advanced civil aircraft wings in the world!:smthumbup
So you don't hold a pilots licence then?

RINGREY said:
I didn't say that the flaps primary function is to slow the aircfaft down! You tell me where I stated that!
Directly above -
RINGREY said:
On landing the pilot will use the flap on the trailing edge to slow the aircraft down
RINGREY said:
The spoilers/air brakes primarily do that, however, as a secondary function they will act to slow down the a/c as a direct result of increased drag.
Now this is correct, however suggesting that a pilot uses flaps to reduce airspeed it not.

RINGREY said:
I think that you pilot friend will agree 100% with me!!!;) Now go back to your tyre kicking!
Which pilot friend? I'm the one with the pilots licence, unlike you! ;-)
 
#29 ·
:lol:

erm i hold a full transport licence.......

a small amount of clarification is required here:

Ground speed, Indicated Airspeed, Rectified Airspeed or True Airspeed?

Which figure would you like......they are all very different ;)
 
#31 ·
let me give you an example:

wind straight down the runway at 50mph

aircraft stall speed with no flap at ground level 70mph

how fast, relative to the ground, does the aircraft need to travel to get or remain airborne?? in this case and assuming no airspeed errors to assess answers on a postcard please?
 
#32 ·
lol lets just assume indicated = True fella.......

but ground speed or airspeed, massive difference, in my case above......

20mph

have u ben ever flown a C152 BACKWARDS over the ground in high wind and full flap? its WIERD!!!!!
 
#37 ·
:lol: im a fixed wing boy ;)

its really most upsetting to the mind to see you moving backwards when really u are flying forwards.......

had touch down speeds of like 10mph before now......vs 60 odd in still conditions.....

ground effect can best be described as a cushion as you flare to land and can catch u right out as a tyro.....

sends you up into the air, and then you kinda stall in almost, massive sink all of a sudden. harsh on the U/C

case of watching how much u flare when u land and its aircraft specific, bigger the wingspan the more pronounced more or less....
 
#39 ·
:lol: im a fixed wing boy ;)

its really most upsetting to the mind to see you moving backwards when really u are flying forwards.......

had touch down speeds of like 10mph before now......vs 60 odd in still conditions.....

ground effect can best be described as a cushion as you flare to land and can catch u right out as a tyro.....

sends you up into the air, and then you kinda stall in almost, massive sink all of a sudden. harsh on the U/C

case of watching how much u flare when u land and its aircraft specific, bigger the wingspan the more pronounced more or less....
It this what helps create pilot induced oscillation - i.e. bouncing down a runway?

Not you mention the flare I can understand totally.

In a heli, if you hover on the spot you create a ground cushion, which reduces power required to maintain static... as soon as you move in any direction you "fall off" that cushion so power required increases, but at the same time as you travel in a direction you start to create translational lift and the rotor disc become more efficient... they don't quite counter act each other so there is some fiddling with power to transition from static to 20kts maintaining fixed altitude above ground.

Also changes in the surface have huge effect, you drop about 20% power in a Robinson R44 moving from grass to concreate in a 5ft hover.
 
#38 ·
i know what you mean about ground effect on the heli, the net effect is like a swash up and over onto the heli blades, can almost push u into the deck (im not heli rated so dont shoot me if im wrong) which is why helis are not just dropped down from height, but almost landed with forward motion to the last moment??
 
#40 ·
What you're describing is Vortex Ring State, where you effectively fly into your own down wash and get sucked into the ground. It's very dangerous as onset --> unrecoverable can be VERY quick.

If you have over 30kts you don't get it at all, hence the foward and down approach we like to take. You need a reasonable descent rate to get it though, the magic numbers tend to be under 30kts and over 500fpm descent rate and you're playing with fire!

+ also, height or airspeed - it's nice to have both, but preferable at least one... so try and keep some airspeed right until the end.
 
#41 ·
yep that kinda suck ya into the deck feeling, im not a million miles away. have been on dual controls on heli and noticed the inputs :mhihi:

but ye that why u see some inexperienced pilots struggle like hell on the flare, uppy downy and believe in a 747 the difference is BIGGER

that said my largest commercial was EMB110. 650hp per PW turbo prop. MENTAL when empty, a DOG when laden.

still managed to pull off regular greasers of landings though :mhihi:

never forget the first commercial landing i did, so smooth the whole 20 odd pax stood up and cheered :lol:

bunch of ****ed up germans :crackup:all wanted to congratulate me when they got off the plane on the best landing they had ever had.....photographs blah blah blah, was hilarious....i have many stories to tell, sham i lost my medical :cry:

was a proper greaser too, hardly knew i had landed myself :D
 
#44 ·
jeez basically this comes down to windspeed and direction and altitude, at ground level and assuming zero wind and above the groiund effect layer:

Vlof of a 747 is about 220 mph.....thats when it physically lifts off, theres a safety factor in that too.

have had to research Vlof though for the 747......its not in my mind :smthumbup

does that answer the OP's question :lol:

VLOF= Calibrated airspeed at which an aircraft first becomes airborne.....
 
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